A bike computer with a cadence meter isn't going to answer his question all by itself without knowing what to do with it. All that will do is give him numbers to look at while he still fumbles his way through every gear combination he has. The key is in understanding ratios and how to use them to produce the most efficient pedaling - maintaining a reasonably high cadence regardless of terrain. Then, if he feels there are gaps in his gearing, where simply shifting to the next available chainring, or the next available cog, is producing too large a change one way or the other, he can look in detail at what ratios are available to him and see where he needs to cross-shift to avoid those gaps.